1. Field of the Invention
The present invention relates to a pneumatic tire, including a tread portion comprising a plurality of block arrays each of which is constituted of a plurality of blocks that are partitioned by a plurality of main grooves extending in a circumferential direction of the tire and a plurality of lateral grooves crossing the main grooves. The pneumatic tire has low rolling resistance and higher resistance against irregular wear.
2. Description of the Related Art
Generally, pneumatic tires have a tread portion formed with a plurality of block arrays, and traction performance is satisfactorily ensured. Such pneumatic tires are excellent in braking, turning and cornering characteristics. However, in such pneumatic tires, a large deformation is generated on the blocks during running, and as the hysteresis loss increases, rolling resistance tends to increase. Also, rigidity difference among the blocks tends to cause irregular wear such as stepped wear.
Japanese Unexamined Patent Publication (Kokai) No. 2006-82633 and Japanese Unexamined Patent Publication (Kokai) No. 4-349004 disclose a pneumatic tire, which has a tread portion including a block array located in a shoulder area thereof; and in one side in a width direction of the tire, a thin rib extends in a circumferential direction of the tire. However, such pneumatic tires disclosed in the documents have problems as described below. That is, the block array generates a rigidity difference between the thin rib side and the opposite side, and when the tire comes into contact with the ground, a larger slip is made on the block located at the opposite side. Irregular wear tends to be generated in such block array. Further, in the pneumatic tires disclosed in the documents, the thin rib is segmented by lateral grooves as viewed in the circumferential direction of the tire. Compared with the case in which a thin rib continuously extends in a circumferential direction of the tire is disposed in a block array, the block rigidity within the block array largely decreases. As a result, deformation amount of the block becomes larger, and as the hysteresis loss increases, the rolling resistance tends to increase in the block array.
Also, Japanese Unexamined Patent Publication (Kokai) No. 2007-50775 discloses a pneumatic tire with a thin rib extending in a circumferential direction of the tire in a central area of a block array as viewed in a width direction. In this pneumatic tire also, a thin rib is segmented by lateral grooves in a circumferential direction of the tire. Therefore, a rigidity difference tends to be generated between the respective blocks in a circumferential direction of the tire causing irregular wear in the block array. Furthermore, in the above pneumatic tire, since the thin rib is segmented as viewed in a circumferential direction of the tire, the rolling resistance tends to increase in the block array.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2001-121927 discloses such a pneumatic tire that an array of one or more ribs is disposed in a tread portion thereof. In a central area of the rib array as viewed in a width direction of the tire, a thin rib, which continuously extends in the circumferential direction of the tire, is disposed to prevent the wear of the rib array. However, in such pneumatic tire, the thin rib is disposed in the rib array. Therefore, the pneumatic tire is not intended to solve the problem; i.e., to reduce the deformation amount of the block and to reduce the rigidity difference among the blocks.